[Usenet: prob. fr. mainstream “puppy pile”] When many
people post unfriendly responses in short order to a single posting, they
are sometimes said to “dogpile” or “dogpile on”
the person to whom they're responding. For example, when a religious
missionary posts a simplistic appeal to alt.atheism, he can expect to be dogpiled.
It has been suggested that this derives from U.S. football slang for a
tackle involving three or more people; among hackers, it seems at least as
likely to derive from an ‘autobiographical’ Bugs Bunny cartoon
in which a gang of attacking canines actually yells “Dogpile on the
rabbit!”.